order_creator.rb

class OrderCreator
  def initialize(order_klass=Order, tax_calculator_klass=TaxCalculator)
  end

  def create(order_params)
    order = order_klass.new(order_params)

    tax_service = tax_calculator_klass.new
    order       = tax_service.calculate(order)

    if order.save
      [:succeeded, order]
    else
      [:failed, order]
    end
  end
end

orders_controller.rb

class OrdersController
  def create
    service = OrderCreator.new

    status, @order = service.create(params[:order])

    if status == :succeeded
      redirect_to @order
    else
      render action: 'new'
    end
  end
end

tax_calculator.rb

class TaxCalculator
  TAX_RATE = 20.freeze # percent

  def calculate(order)
    order.tax   = order.subtotal * (TAX_RATE / 100)
    order.total = order.subtotal + order.tax

    order
  end
end